The present invention relates to an apparatus for driving an electromagnetic valve used for supplying fuel to an engine of a vehicle.
A conventional electromagnetic valve driving apparatus of this type is disclosed in U.S. Pat. No. 4,605,983 (JP-B2-4-42805). When this apparatus is applied to a high-pressure fuel pump, a conduction current flowing through a coil employed in an electromagnetic valve is driven to reach a predetermined magnitude Ip before a predetermined period of time lapses after the start of the valve driving so that the movable body of the valve may be actuated quickly. Thereafter, the conduction current is reduced to a small holding magnitude Ih (&lt;Ip) which is large enough only for holding the movable body of the valve at the current position. As a result, a good driving response of the electromagnetic valve or the operating response of the valve body can be attained by merely supplying a reduced conduction current or a reduced driving current.
In a state where the engine is being started or various electric loads are turned on under a low engine speed condition such as an idling, a battery voltage drops due to the operation of a starter motor or the electric loads. In addition, the conduction-holding period becomes longer at a very low engine speed. It is therefore likely that the battery voltage substantially decreases even during the conduction-holding period. As a result, according to the above apparatus, the amount of fuel supplied to the engine during the start time of the engine is not sufficient and the start characteristic of the engine is lessened.
U.S. Pat. No. 4,605,983 also discloses to lengthen a predetermined period of limiting the conduction current to a peak current as the voltage of the battery decreases. However, this is not useful for solving the problem of a drop in battery voltage, which occurs in the conduction-holding period following the predetermined period.
The above problem occurs irrespective of the type of the electromagnetic valve, that is, a normally closed-type or a normally open-type. In addition, the above problem is also encountered in the case of an electromagnetic valve employed as a fuel injector (a fuel injection valve) for supplying fuel to the engine. In this case, if the duration of the operating time of the electromagnetic valve becomes too short, the amount of fuel supplied to the engine is not sufficient by a quantity determined by a decrease in operating-time duration.
In another conventional apparatus disclosed in JP-A-8-4576, a current supplied to a fuel injection valve is set at a value higher than that of the ordinary operation only at the first injection at the time the engine is started. However, since it is still impossible to improve the start characteristic of the engine accompanying a drop in battery voltage, because the voltage drop is not predictable. In addition, in order to supply a current with a magnitude greater than that of a normal condition through the coil at a low battery voltage, a voltage raising circuit must be provided.